Please help with mounting plate adapter

I have a old Peerless articulating arm wall mount(model # PLA-1) that was hold my 10yr old Pioneer 50' plasma. I'm tring to use the same mount for my new 55' LG 9600. However the screws don't match up from the old mounting plate to the new TV. I need a plate for the new tv that measures 400*400 VESA that will mount to the old Peerless plate arm. Does anyone know what if any plate adaptor I need. Thanks

In case anybody's interested. I'm told this is what I need. Peerless PLP-V4X4 Custom Interface Bracket